1. Technical Field of the Invention
The present invention relates to a howling detection circuit which is suitable for use in an audio mixer or the like.
2. Description of the Related Art
When howling occurs, a mixer or the like has to specify the source of the howling promptly and perform a level control, frequency characteristic control or the like in association with the specified source. Various techniques have been proposed with respect to a howling detection method. For example, disclosed in Japanese Patent Laid-open Publication No. Heisei 4-277977 is a technique that detects howling by measuring the levels of respective frequency components of an audio signal, comparing the measured level of each of the frequency components with those of the other frequency components and determining from the comparison whether a frequency component of an unnaturally high level is present in the frequency components of the audio signal.
However, the above-mentioned technique is disadvantageous in that complex processes, such as frequency component detection, etc., are required, resulting in complexity in configuration of a howling detection circuit and increase in cost when the howling detection circuit is installed in a mixer or the like. Particularly, because the mixer mixes audio signals of a plurality of input channels to generate an output signal, it must have the howling detection circuit for each of the input channels and, when howling occurs, promptly specify one of the input channels as the source of the howling. In this configuration, howling detection circuits of a number corresponding to that of the input channels are required, causing increase in cost.
Assume that an audio signal inputted from a microphone is amplified by an amplifier, outputted through a speaker and then fed back to the microphone. In this case, if a frequency component whose gain in one circulation of the input audio signal to the microphone exceeds “1” is present, howling occurs in that frequency component. Once the howling occurs, it continues ceaselessly as long as there is no change in the position relationship between the microphone and the speaker or the gain of the amplifier. Meanwhile, assume that a human voice or natural musical instrument's sound is inputted through the microphone. In this case, a “node” may be present in the voice or sound. This node may be regarded as a timing at which the variable level of the input audio signal falls during the utterance of the voice or sound. In this regard, whether or not the howling has occurred can be easily determined on the basis of the presence or not of the “node” in the microphone input.